Carbonaceous soot may be a bi-product of some combustion processes. For example, carbonaceous soot may be produced by some diesel engines during higher engine load conditions. More recently, gasoline engines have incorporated directly injecting fuel into engine cylinders to improve engine performance and fuel economy. However, directly injecting fuel to engine cylinders has also increased the possibility of producing carbonaceous soot in gasoline engines. As a result, some manufacturers are considering placing particulate filters within the exhaust systems of gasoline engines.
Particulate filters can hold carbonaceous soot, but over time, the soot accumulated within the particulate filter can reduce exhaust flow through the exhaust system. Consequently, engine back pressure may increase, thereby reducing engine efficiency and fuel economy. Buildup of soot within the particulate filter can be controlled by periodically oxidizing the soot. Soot trapped in a particulate filter can be oxidized by elevating the temperature of engine exhaust gas flowing into the particulate filter and providing excess oxygen for oxidation. However, elevating engine exhaust temperatures may reduce engine fuel economy since the engine may be operated less efficiently to increase exhaust gas temperatures. Therefore, it may be desirable to limit soot purging or regeneration of the particulate filter to conditions where the particulate filter holds an amount of soot that warrants oxidation.
One way to determine whether or not timing is desirable for oxidizing soot held within a particulate filter requires measuring exhaust pressure upstream and downstream of the particulate filter. If a pressure difference greater than a threshold amount develops between the upstream and downstream pressure measurements, it is determined that there is sufficient soot mass for the oxidation process. Although determining a pressure difference within the exhaust system may be possible, adding pressure sensors to the exhaust system raises system cost. In addition, pressure sensors may not be as durable in the exhaust system as compared to other types of sensors.
The inventor here has recognized the above-mentioned disadvantages and has developed a method for determining operating a particulate filter, comprising: estimating soot mass oxidized from a particulate filter via first and second oxygen sensors; and indicating degradation of the particulate filter when a difference between the estimated soot mass oxidized from the particulate filter and a desired soot mass held in the particulate filter is greater than a threshold soot mass.
A mass of soot oxidized within a particulate filter can be determined in response to output of oxygen sensors. In one example, a mass of soot oxidized in a particulate filter is determined from an amount of oxygen that is consumed during soot oxidation. Oxygen sensors are typically included in the exhaust systems of gasoline engines to improve air-fuel control and three-way catalyst efficiency. Thus, a mass of soot held within a particulate filter may be determined via oxygen sensors that are in the engine exhaust for determining engine air-fuel control. Consequently, cost for a system to control particulate filter soot can be reduced since oxygen sensors can be used for more than a single purpose.
The present description may provide several advantages. In particular, the approach can reduce particulate filter system cost since oxygen sensors determine a mass of soot within a particulate filter rather than pressure sensors. In addition, particulate filter leakage may be determined with the approach. Further, the approach compensates for differences in oxygen sensor output to improve stored soot estimates whereas output of pressure sensors and particulate sensors may be influenced by the presence of the particulate filter.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.